Method of annealing metal



April 30, 1940. H. UNCKEL 2,199,326

METHOD OF ANNEALING METAL Filed May 3. 1937 Patented Apr. 30, 194i)UNITED STATES 2,199,326 l METHOD OF ANNEALING METAL Hermann Unckel,Finspong, Sweden Application May 3, 193i, Serial No. 140,565

. In Sweden May 5, 1936 i Claim.

This invention relates to a method of producing smooth surfaces onmetals and metal articles in annealing them to restore their ductilitysubsequent to a cold working process by which they have been renderedhard. The usual practice up till now in annealing metal articles with aview to restore their ductility has been to anneal the articles byheating them to a temperature above the recrystallisation point of themetal and, after cooling, remove the oxide film formed duringtheannealing process by pickling the articles in an acid bath. -By virtueof the oxidation taking place on the metallic vsurface during theannealing and by the action of the strong pickling agent required forremoving the oxide film, the metallic surface becomes dull.

Sometimes, however, it is ,desired to give the annealed article a smoothand bright surface.

' In the recent years several brightening methods brass, none of thebrightening methods hitherto commercially employed, has given reallysmooth or bright surfaces. In the annealing of brass according to knownpractice the'zinc which forms one of the constituents of this alloy isoxidized exceedingly easily so that the small amounts of oxygen whichare present in the furnace atmosphere, even in case of a closed furnace,form surface oxides. Moreover, the zinc, when heated to hightemperatures, has a rather high vapor pressure so that zinc willevaporate and, in part, again condense on the metallic surface, makingthe same dull,

According to the present invention it is possible in annealing processesto obtain completely smooth and bright surfaces even in case of verysensitive metals and alloys, as for instance, brass. The inventionresides in the combination with the well-knownannealing process which iscarried out in a substantially inert atmosphere, from which the oxygenof the air is excluded, of a pickling process carried out by the use ofweak pickling agents the acidity of which is suflicient to clean theoxides from the metallic surface but insufficient to materially attackthe, metallic surface proper. The novel method wil hereinafter bedescribed as applied to brass,.

th ugh it is to be noted, that it is not restricted and furnaces tocarry said methods into effect- (Cl. 148-4) q to this alloy .but may beapplied to advantage to other metals and allows which are of such anature as to be easily attacked by the oxygen of the air and strongpickling agents. 3

"In the accompanying drawing, Fig. 1 is a diagrammatic verticalsectional view ofa plant, including a muffle furnace, for carrying outthe method in a discontinuous way. Fig. 2 is a side elevation, partly insection, of a plant including a muflle furnace for continuous operation.

In Fig. 1, reference numeral I indicates the furnace wall, 2 is thematerial to be annealed, and 3 indicates. the end of the mufile,immersed in a water mass 4. After the annealing process is completed,the material 2 is immersed in the water 4 by means of a raisingandlowering or immersion device indicated at 5. This furnace allows only adiscontinuous operation. Fig. 2 shows a furnace for continuousoperation. The muffle furnace is indicated by the numeral l. The muflleends 3 3 are bent and each immersed in a water mass 4 4 In this case,the material under treatment is conveyed continuously through the waterseals and the furnace by means of a conveying chain or a conveying band6.

In carrying out the annealing process-accorde ing to a preferred form oftheinvention, predetermined'charges of the material to be treated areplaced in a closed mufile furnace, as illustratedin Fig. 1 of thedrawing, filled with a dry, inert gas or with steam, if desired, mixedwith an inert gas so that under,all conditions the oxygen of the airwill be excluded as perfectly as possible. In many cases, however, it ispreferred to employ a type of furnace in which the ends of the muffleare bent downwards and immersed in water, as illustrated in Fig. 2, in

order to close the muille proper, within which the annealing process iscarried out, against the atmosphere by water seals. In this latter case,the material under treatment is conveyed continuously through the waterseals and the furnace by means of the conveying chain or conveying band6. In case of drawn or rolled material of great lengths, the materialmaybe pulled directly through the water seals and the muflle. Brass willnot be as bright as glass by such an annealing treatment but willpresent a discolored surface which may be caused by a certain oxidationresulting from'reactions with mineral constituents of the water used aswater seals or by oxygen dissolved in the water or liberatedtherefromandto some extent also by a slight evaporation of zinc. It was. therefore,

up till now considered impracticable to give brass a smooth and brightsurface by an annealing process. According to this invention, thediscoloring film formed on the annealed material is removed by picklingthe material in a very weak pickling bath containing, for instance,tartar or tartaric acid, or extremely dilute hydrochloric acid, aceticacid, sulphuric acid or other very dilute acids (of a concentration of 2per cent., for instance, or less), and then washing the material anddrying it in the usual-Way. By this treatment, the surprising effect isobtained that the surface is perfectly smooth and as bright as glass.Instead of effecting the pickling process after the discharge of thematerial from the furnace, the pickling agent may be added to thatportion of the water which forms the water seal at that end of themuflie where the material is discharged, whereby a separate picklingoperation is eliminated.

In the oxidizing annealing of brass and similar materials hithertocommonly employed, an effective pickling treatment must always beefiected subsequent to the annealing treatment, to remove thecomparatively deep oxide layer. In case of brass, a 20 per cent.sulphuric acid solution is usually employed at a temperature of about50-'70 C. If a weaker bath wereused, then the pickling time would beuneconomically long and still no smooth and bright surface would beobtained, because the metallic surface covered by the oxide layer wouldalready be rough and uneven as a result of the extended oxidation causedby the annealing, and the unevenesses thus produced cannot be removed bya pickling process.

The activityor acidity of the pickling liquor' employed according tothis invention, is only about a tenth or a twentieth of that hithertoused in effecting a pickling subsequent to an oxidizing annealingprocess. As an example, a

bath containing 2 per cent. of tartaric acid, the

.and the temperature of the pickling bath and the duration of thepickling action so as to only remove the miscolouring surface layer ofthe annealed material while not attacking the metallic surface proper.By this means the smoothness and brightness of the metal, after thepicklingprocess is finished, will be very close to that of the materialprior to the annealing process. In order to express this degree ofsmoothness or brightness by figures it might be mentioned thatinvestigations made by Messrs. Kenworthy and Waldram (Journ. Inst. Met.,of 1934, page 247 if) and by Mr. R. Toussaint (Rev. dAluminium, of 1934,page 2403) have proved that the highest smoothness and brightness (asmeasured by the reflexion from a brass surface at a 45 angle ofincidence of light) at the most perfect polishing amounts to about 95per cent. of the incident light. On the contrary, a dull reflectingsurface has a coefficient of reflexion not higher than up to 10-20 percent. Smooth rolled brass might have a reflexion coefiicient up to about50-60 per cent. An annealing and pickling treatment according to thehitherto employed methods yields, when applied to brass, a surfacereflexion of 10-20 per cent., whereas a treatment according to thepresent invention will give the brass surface a smoothness andbrightness very close to that of the smooth rolled material prior to theannealing treatment.

The technical advantages gained by the invention may be briefly summedup as follows. Hitherto, in order to obtain a smooth and bright surface,a brightening annealing treatment according to one of the well-knownmethods has been employed whereby satisfactory results may be obtainedin case of certain materials. As to other kinds of material, as brass,the bright annealing treatment is entirely abandoned, as by thistreatment it was impossible to obtain a glassy surface, and insteadthereof a usual oxidizing annealing treatment and a subsequent strongpickling .has been adopted. According to the invention I use a brightannealing treatment and a subsequent weak pickling process in case of.and brightness which up till now was not obtain- The method ofproducing bright surfaces in the annealing of metal containing acomponent having a strong tendency to oxidize at annealing temperatureand which has been rendered hard by a cold working process whichcomprises passing the material to be annealed continuously through awater seal into a muflie furnace and through a practically inertatmosphere therein, free from oxygen to reduce oxidation as far aspossible, heating thematerial to the annealing temperature during thispassage, and passing the annealed material out of the furnace through ahydraulic seal formed by a weak pickling agent of an acidity sufficientto dissolve metallic oxides but insuflicient to attack the metallicsurface proper.

HERMANN UNCKEL.

